Fluorescent metabolic substances are known from the field of biotechnology which either become concentrated exclusively in certain regions such as, for example, tumors, inflammations, and other specific focuses of a disease, thus being present only locally, or are distributed throughout the body but are activated in terms of their fluorescent property exclusively in certain regions as a result of, for instance, enzyme activity that is peculiar to a tumor. Owing to their fluorescent property, said substances can be employed as markers or marking substances so that a specific, for example already pathological area can be marked or, as the case may be, can be registered at all. A region fluorescently marked in this way is detected by irradiating said region with light of a specific excitation wavelength of the fluorochrome then detecting the emitted light in the corresponding emission wavelength of the fluorogen.